1. Field of the Invention
The present invention relates to WLAN (Wireless Local Area Network) communication devices and corresponding methods and integrated circuit chips, and in particular to the filter tuning in such WLAN communication devices.
2. Description of the Related Art
A wireless local area network is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN. Using radio frequency or infrared technology, WLAN systems transmit and receive data over the air, minimizing the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Today, most WLAN systems use spread spectrum technology, a wideband radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade off bandwidth efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to 802.11b that allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. Further extensions exist.
Examples of these extensions are the IEEE 802.11a, 802.11b and 802.11g standards. The 802.11a specification applies to wireless ATM (Asynchronous Transfer Mode) systems and is used in access hubs. 802.11a operates at radio frequencies between 5 GHz and 6 GHz. It uses a modulation scheme known as OFDM (Orthogonal Frequency Division Multiplexing) that makes possible data speeds as high as 54 Mbps, but most commonly communications take place at 6 Mbps, 12 Mbps or 24 Mbps. The 802.11b standard uses a modulation method known as CCK (Complementary Code Keying) which allows high data rates and is less susceptible to multipath propagation interference. The 802.11g standard can use data rates of up to 54 Mbps in the 2.4 GHz frequency band using OFDM. Since both 802.11g and 802.11b operate in the 2.4 GHz frequency band, they are completely interoperable. The 802.11g standard defines CCK-OFDM as optional transmit mode that combines the access modes of 802.11and 802.11b, and which can support transmission rates of up to 22 Mbps.
In any transmit mode, a WLAN communication device, i.e. transmitter, receiver or transceiver, needs to filter the communication signal in order to eliminate unwanted interference and noise. In a WLAM receiver, filtering of a received communication signal is accomplished to remove signals with frequencies outside of a determined frequency range to avoid overloading of the receiver, and in particular any signal falling within the image frequency, i.e. the frequency that results, when downconverted by a mixer, to the same intermediate or baseband frequency as the desired communication signal. In a WLAN transmitter, filtering is used to ensure that the transmitter only emits signals within the allowed frequency range by removing other spurious signals that may be introduced into the communication signal, e.g. due to imperfections in the transmitter circuitry.
In order to achieve the desired filtering, there is a need to adjust such filters after manufacturing by initially tuning them to the desired frequency response. This includes tuning the cut-off frequency (or frequencies) of the filter above or below which signals can pass the filter. Since many WLAN communication devices operate at a number of different channels in a given frequency band, continuous tuning of the cut-off frequency during operation of the WLAN communication device is also required. Especially when frequency hopping techniques are used, the tuning circuitry needs to allow for quickly adapting the cut-off frequency to a new channel frequency. Further, continuous tuning is needed for compensating for a cut-off frequency drift caused, e.g., by temperature coefficients of filter components that change in the ambient or operating temperature.
Many conventional WLAN communication systems use a master-slave tuning technique for achieving real time cut-off frequency tuning. In the master-slave architecture, a master oscillator is implemented using circuits similar to those employed within a slave filter to be tuned. Both circuits receive the same frequency control input which is derived by phase locking the master oscillator to an external reference. Thus, when the frequency of the master oscillator is set, the passband frequency of the slave filter is properly tuned. However, substantial additional circuitry is required for implementing the master-slave tuning technique. Therefore, conventional WLAN communication devices often suffer from the problem of increased power consumption. In addition, those WLAN communication devices have the disadvantage of causing high manufacturing and product costs.
In order to overcome the problems arising in master-slave tuning systems, the self-tuning technique was developed. In this approach, the filter is periodically taken offline and tuned directly. Yet, since filtering of the communication signal is interrupted while the filter cut-off frequency is being tuned, prior art WLAN communication devices applying the self-tuning technique often have difficulties in achieving efficient data rates. Further, the filtering accuracy in those devices is decreased due to, e.g., cut-off frequency drift, during the time interval between the individual tuning interruptions.